I just moved several mailboxes to a new DB. The total size of the mailboxes being moved was 43 GB but the EDB file grew to 69 GB. Is this normal? What causes the additional growth? Will we get any of that space back through online defragmentation?

You'll get logical space back with online defrags, but not physical. What you are seeing is not too unusual. The mailbox sizes listed in EMC do not necessarily reflect reality. What version of Exchange is this?

Exchange 2007 SP1.

Ok, just wondering. Exchange 2003 edbs can grow as you move mailbox and items are moved from the source stm files. Regardless, what you are seeing is not that unusual in my experience. Note that Events 1221 will tell you how much free space is avail in the store: http://support.microsoft.com/kb/186291 I also recommend upgrading to SP2 RU3. :)

Okay, thanks for the sanity check Andy.